Add inline_br option for linebreaks in inline elements

[TytoCapensis]

Some HTML tables (and generally, other inline elements) may use <br> in table cells. For example:

<table>
<tr>
	<th>User</th>
	<th>Email</th>
</tr>
<tr>
	<td>John Doe</td>
	<td>john.doe@example.com</td>
</tr>
<tr>
	<td>Jane Doe</td>
	<td>jane.doe@example.com<br>jane.doe@example.org</td>
</tr>
</table>

Currently, line breaks with <br> in inline elements are replaced by spaces. The table above is converted to this by markdownify:

| User | Email |
| --- | --- |
| John Doe | john.doe@example.com |
| Jane Doe | jane.doe@example.com jane.doe@example.org |

This is acceptable, but it may lead to confusion in some cases.
There is no official Markdown way to have line-breaks inside cells, but the de-facto standard is to use <br> to manage this:

| User | Email |
| --- | --- |
| John Doe | john.doe@example.com |
| Jane Doe | jane.doe@example.com<br>jane.doe@example.org |

(Note that this also works in GitHub's Markdown)

This pull request adds a inline_br / --inline-br option to keep all inline <br> tags. (This option is disabled by default to not disrupt existing behaviors)

All tests are passing. If I force inline_br on all tests, 2 are failing (because of a <br> conversion, which is expected). Feel free to suggest how this could be integrated in tests!